1. Field of the Invention
The present invention relates to seed crystals for pulling a single crystal and methods using the same and, more particularly, to seed crystals for pulling a single crystal which are used in pulling a single crystal of silicon or the like by a method such as the Czochralski method (hereinafter, referred to as the CZ method) and methods for pulling a single crystal using the same.
2. Description of the Prior Art
At present, as the majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like, silicon single crystals pulled by the CZ method have been used. FIG. 1 is a diagrammatic sectional view of an apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral 11 represents a crucible.
The crucible 11 comprises a bottomed cylindrical quartz crucible 11a and a bottomed cylindrical graphite crucible 11b fitted on the outer side of the quartz crucible 11a. The crucible 11 is supported with a support shaft 18 which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater 12 of a resistance heating type and a heat insulating mould 17 arranged around the heater 12 are concentrically arranged around the crucible 11. The crucible 11 is charged with a melt 13 of a material for forming a crystal which is melted by the heater 12. On the central axis of the crucible 11, a pulling axis 14 made of a pulling rod or wire is suspended, and at the front thereof, a seed crystal for pulling a single crystal 35 (hereinafter, simply referred to as a seed crystal) is held by a seed chuck 14a. These parts are arranged in a water cooled type chamber 19 wherein pressure can be controlled.
A method for pulling a single crystal 36 using the above-mentioned apparatus for pulling a single crystal is described below by reference to FIGS. 1 and 2. FIGS. 2(a)-(d) are partial magnified front views diagrammatically showing the seed crystal and the vicinity thereof in part of the steps in pulling a single crystal.
Although it is not shown in FIG. 2, the material for forming a crystal is melted by the heater 12. The pressure in the chamber 19 is reduced and is maintained for a period of time so as to sufficiently release gas contained in the melt 13. Then, an inert gas is induced into the chamber 19 so as to make an inert gas atmosphere under reduced pressure within the chamber 19.
While the pulling axis 14 is rotated on the same axis in the reverse direction of the support shaft 18 at a prescribed speed, the seed crystal 35 held at the front of the pulling axis 14 is caused to descend and is brought into contact with the melt 13 so as to make the seed crystal 35 partially melt into the melt 13. Then, the single crystal 36 begins to be pulled (hereinafter, referred to as the seeding step) (FIG. 2(a)).
In making a crystal grow at the front of the seed crystal 35, the pulling axis 14 is pulled at a higher speed than the below-described speed of the formation of the main body 36c. The crystal is narrowed to have a prescribed diameter, leading to the formation of a neck 36a (hereinafter, referred to as the necking step) (FIG. 2(b)).
By slowing down the pulling speed of the single crystal 36 (hereinafter, simply referred to as the pulling speed), the single crystal 36 is made to grow to have a prescribed diameter, leading to the formation of a shoulder 36b (hereinafter, referred to as the shoulder formation step) (FIG. 2(c)).
By pulling the single crystal 36 at a fixed rate, the main body 36c having a uniform diameter and a prescribed length is formed (hereinafter, referred to as the main body formation step) (FIG. 2(d)).
Although it is not shown in FIG. 2, in order to prevent induction of high density dislocation to the single crystal 36 by a steep temperature gradient, the diameter thereof is gradually decreased and the temperature of the whole single crystal 36 is gradually lowered, leading to the formation of an end-cone. Then, the single crystal 36 is separated from the melt 13. Cooling the single crystal 36 is at the end of the pulling of the single crystal 36.
One of the most important steps in the pulling of the single crystal 36 is the above-mentioned necking step (FIG. 2(b)), and the object of the necking step is described below. In the seeding step (FIG. 2(a)), the bottom portion 35a of the seed crystal 35 is preheated to some extent and is dipped into the melt 13. There is a difference of 100.degree. C. or more between the preheating temperature (about 1300.degree. C. and less) and the temperature of the melt 13 (about 1410.degree. C.). Therefore, in dipping the seed crystal 35 into the melt 13, the seed crystal 35 has a steep temperature gradient, leading to the dislocation caused by a thermal stress at the bottom portion 35a of the seed crystal 35. It is necessary to make the single crystal 36 grow after excluding the dislocation which inhibits single crystal growth. Since the dislocation generally tends to grow in the vertical direction to the growth interface of the single crystal 36, the shape of the growth interface (the front plane 36a' of the neck 36a) is made to be downward convex as shown in FIG. 2(b) in the necking step, so as to exclude the dislocation.
In the necking step, the faster the pulling speed is made, the smaller the diameter of the neck 36a can be made, so that the shape of the growth interface can be a downward convex having a smaller diameter. As a result, the dislocation can be inhibited from propagating and can be efficiently excluded.
In the above conventional method for pulling a single crystal, the seed crystal 35 having a diameter of about 12 mm has been generally used in order to pull the single crystal 36 having a diameter of about 6 inches and a weight of 80 kg or so. The larger the diameter of the neck 36a is, the more safely the single crystal 36 can be held, while the smaller the diameter of the neck 36a is, the more efficiently the dislocation can be excluded. In order to meet both of the requirements, the neck 36a having a diameter of 3 mm or so is selected. Recently, however, in order to produce a more highly integrated semiconductor device at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal 36 having a diameter of 12 inches (300 mm) and a weight of 300 kg or so is desired. In this case, the neck 36a having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal 36 and breaks, resulting in the falling of the single crystal 36.
In producing the above heavy single crystal 36, the diameter of the neck 36a needs to be about 6 mm in order to prevent the occurrence of troubles such as a fall of the single crystal 36 and to pull the single crystal 36 safely, which is calculated from the silicon strength (about 16 kgf/mm.sup.2). However, when the diameter of the neck 36a is 6 mm or more, the dislocation occurs in dipping the seed crystal 35 into the melt 13 and cannot be sufficiently excluded.